In the operation of a typical GMR head device, a moving magnetic storage medium, typically a disk, is placed near the pole-tips of the GMR head. During the read operation, the changing magnetic flux from magnetized regions in the moving storage disk induces a changing magnetic flux in the pole-tips and the gap between them. The magnetic flux is carried through the pole-tips and yoke-shaped core and around spiral conductor coil winding turns located between the yoke arms. The changing magnetic flux induces an electrical voltage across the conductor coil. The electrical voltage is representative of the magnetic pattern stored on the moving magnetic storage disk. During the write operation, an electrical current is caused to flow through the conductor coil. The current in the coil induces a magnetic field across the gap between the pole-tips. A fringe field extends into the nearby moving magnetic storage disk, inducing (or writing) a magnetic domain in the magnetic storage disk. Impressing current pulses of alternating polarity across the coil causes the writing of magnetic domains of alternating polarity in the storage disk.
The GMR head is normally attached to a substrate, the head and substrate together forming a slider. The substrate includes aerodynamic surfaces that cause the slider to “fly” over the moving disk.
As the recording density of the magnetic domains in the magnetic disks increases, the “flying height” of the GMR heads has become lower. The reduced flying height is necessary to enable the head to read the data bits stored on the disk effectively and without interference or crosstalk from adjacent data bits.
The lessening in the flying height has created a number of problems in the fabrication of the GMR heads. One of these problems relates to the thermal properties of the layers that together make up the head. In particular, the head tends to heat up by friction with the supporting layer of air as the head “flies” over the disk, and the constituent layers expand as this happens. This expansion increases the risk that the head will contact or “crash into” the disk, thereby damaging the head, the disk, or both, and that stresses will be created between the layers in the head.
Typically, the top layer in the head is a relatively thick “overcoat layer” that is formed of alumina (Al2O3). One problem with alumina is that its coefficient of thermal expansion (CTE) of 6 μm/m/° C. is relatively high, which creates a temperature-induced protrusion at the air-bearing surface (ABS) when the head heats up. Other materials with lower CTEs might be desirable as substitutes for alumina in the overcoat layer, but in many cases these other materials present manufacturability problems.
U.S. Pat. No. 5,643,259 to Sone et al. describes the formation of a recess in the overcoat layer at the trailing edge of the slider, which, it claims, prevents the temperature-induced protrusion from extending “above a predetermined level of the surface facing the disk” (col. 2, lines 50-51). Alumina is used for the overcoat layer, however, so Sone et al. are limited to the relatively high CTE of alumina. Published European Patent Application No. 0627732 A1 teaches an overcoat layer made of silicon dioxide or silicon nitride, both of which have a CTE less than alumina, but it fails to teach a technique for overcoming the fabrication problems presented by the use of these materials, namely, that they tend to chip or crack when the GMR head elements in a wafer are separated from each another by sawing.
Accordingly, what is needed is a material for use in the overcoat layer that has a CTE lower than alumina and yet can readily accommodate to the fabrication process.